The invention relates in general to the field of cooled photovoltaic systems, and more specifically to photovoltaic systems having photovoltaic cells arranged side-by-side to form an array of photovoltaic cells, and further to methods for cooling such photovoltaic systems.
The following definitions are assumed throughout this description.
Photovoltaics (PV) describes the generation of electrical power by converting solar radiation into direct current electricity through semiconductors exhibiting the photovoltaic effect.
A photovoltaic cell (or PV cell, also “solar cell” or “photoelectric cell”) is a solid state device that converts energy of light directly into electricity by virtue of the photovoltaic effect.
A photovoltaic array or module (also “solar module”, “solar panel” or “photovoltaic panel”) is an assembly of connected photovoltaic cells.
A photovoltaic system typically includes at least one module of photovoltaic cells, an inverter and interconnection wiring.
A thermal collector (also “solar thermal collector”) collects heat by absorbing radiation, typically solar radiation.
A heat exchanger or heat coupler is a device or a piece of equipment to efficiently transfer heat from one medium to another.
A heat sink is a heat exchanger that serves to cool a device (such as an array of photovoltaic cells) by dissipating heat from the device into another medium.
Concentrated solar power (also “concentrating solar power” or CSP) systems use mirrors or lenses that concentrate radiative flux of a large area onto a small area, such that electrical power (also “power”) can be produced when concentrated light is converted to heat, which drives a heat engine (e.g., a steam turbine) connected to a power generator. Common forms of concentration are parabolic trough, dish, concentrating linear Fresnel reflector and solar power tower.
Concentrated photovoltaic (CPV) systems use optics (e.g., lenses) to concentrate a large amount of sunlight onto a small area of photovoltaic materials to generate electricity. Concentration allows for usage of smaller areas of solar cells.
CPV should not to be confused with CSP. In CSP concentrated sunlight is converted to heat, and then heat is converted to electricity. In contrast, in CPV concentrated sunlight is converted directly to electricity using the photovoltaic effect.
Photovoltaic thermal hybrid solar collectors (also “hybrid PV/T systems” or PVT) are systems converting solar radiation into thermal and electrical energy. Such systems combine photovoltaic cells, which converts photons into electricity, with a solar thermal collector, which captures the remaining energy by removing heat from the PV module. Two categories of PVT collectors are generally known, namely, PV/T fluid collectors and PV/T concentrators.
In PV/T fluid collectors (air or liquid), which are typically water-cooled, use is generally made of thermally conductive metal piping or plates attached to the back of a PV module. The working fluid is typically water or a water-glycol mixture. The heat from the PV cells is conducted through the metal and absorbed by the working fluid, which assumes that the working fluid is cooler than the operating temperature of the cells. In closed-loop systems this heat is either rejected to ambient or transferred at a heat exchanger, where it flows to its application for further usage. In open-loop systems, the working fluid is not recirculated to the PV cells following rejection or further usage of heat.
In PV/T concentrators (CPVT), a concentrating system is provided to reduce the amount of solar cells needed. CPVT can reach very good solar thermal yield per unit PV cell area compared to flat PV/T collectors. However, main obstacles to CPVT are to provide sufficient cooling of the solar cells and a durable tracking system.
A disadvantage of PV systems compared to other energy sources is the intermittent nature of the direct solar radiation. This leads to intermittent power delivery, which has much less value than on-demand power and may lead to grid instabilities. Storage of electrical energy in, e.g., batteries is prohibitively expensive so that the cost for a storage unit for a full day production may cost more than the solar power station. Concentrated solar power systems (CSP) can store the collected heat and produce electricity on demand until the stored heat is dissipated. Concentrated photovoltaic (CPV) power plants have higher conversion efficiencies than PV and CSP systems. Still, CPV systems are subject to intermittent power production. In addition, their efficiency depends on the performances of the PV cells composing the array, or module, and of the cooling system used to cool the PV cells. Concentrated photovoltaic thermal (CPVT) systems have a higher system efficiency because of the added usage of heat.
CPV systems aim at achieving high geometrical concentrations of solar irradiance on PV cells, typically in the order of 500-3000 suns. Such concentrations are typically enabled by faceted mirrors, focusing light on one single focal plane. However, the mirror topology is never perfect. The varying focal points and acceptance apertures of the individual mirrors lead to a non-homogenous illumination pattern on the focal plane. Thus, optical mixers may be used, which aim at homogenizing the illumination pattern. This, however, reduces the concentration efficiency. An example of a circular illumination pattern is shown in FIG. 3A. An example of a homogenized illumination pattern is shown in FIG. 4A. Color contour plots are used, and the corresponding 3D plots are shown, in each case.
The concentrating optics used in CPV systems generally result in non-uniform illumination on the PV cell surface. A more uniform illumination would come at a cost of lower efficiency, because more optical elements need be integrated. To reduce cost of packaging and cooling, arrays of PV cells (placed side-by-side) are sometimes designed with a common support structure which provides electrical interconnection and cooling. However, in this configuration the PV cells in the array are exposed to different illumination and therefore exhibit different electrical output characteristics. In order to have the same electrical output for each PV cell, several CPV solutions rely on pairing a single optical element with a single PV cell (point-focus systems).
Compared to point-focus systems, dense array systems use closely packed PV cells, which involve several cells per concentrating element. Such solutions may thus offer a cost advantage. In dense array systems, heat generation may be higher than for point-focus systems because there is less surface area per cell for heat dissipation. With appropriate thermal management, the heat generated in a dense array system can be used for polygeneration (i.e., production of electricity, heat and additional resources like, e.g., potable water or air-conditioning), which results in improved cost-performance of the overall system.
Efficient cooling devices have been designed, such as described in WO 2013/144750 A1, where the cooling device provides a thermal resistance which is very low and uniform over a large area, onto which PV cells can be attached. Such a cooling device provides a means to keep PV cell temperatures below a required threshold and, this, even at high concentrations of solar radiation. Furthermore, due to its low thermal resistance, this cooling device allows the generated heat to be recovered at high temperatures, which further allows exploiting polygeneration.